Qt - Powerful, flexible and easy to use, Qt will help you not only meet your tight deadline, but also reduce the maintainable code by an astonishing percentage.
SDL - Simple DirectMedia Layer is a cross-platform multimedia library designed to provide low level...
wxWidgets - wxWidgets: Cross-Platform GUI Library
raylib - Cross-platform, open-source game development library in C.
FLTK - Fast Light Toolkit - Fast Light Toolkit (FLTK)
SFML - SFML provides a simple interface to the various components of your PC, to ease the development of games and multimedia applications. It is composed of five modules: system, window, graphics, audio and network.